The PD-1:PD-L immunoregulatory pathway has major roles in controlling exhausted/dysfunctional T cells and this pathway has become a major clinical target in cancer and chronic infection. The role of this pathway in acute infections is unclear. Available data are conflicting, but have focused on effector CD8 T cell expansion. Our new exciting preliminary data identify a novel and major role for the PD-1:PD-L pathway as a key integrator of signals that result in optimal CD8 T cell memory quality. Specifically, we have found that the PD-1 pathway shapes effector and memory CD8 T cell differentiation during acute influenza (Flu) infection. These findings give impetus to investigating how PD-1 and its ligands control CD8 T cell differentiation following acute infections. Thus, we will define mechanisms by which the PD-1 pathway regulates effector CD8 T cell differentiation and development of memory CD8 T cells during Flu infection. There is a tremendous need for new or improved vaccines against respiratory pathogens such as Flu. However, it has been challenging to harness the potential of T cell-based vaccines for respiratory viral infections partly due to limited knowledge of how to effectively induce the most protective types of memory T cells. Determining how PD-1 promotes CD8 T cell memory may generate a better understanding of protective immunity to Flu and enable novel vaccine strategies for other respiratory viruses. In Project 2 we will interrogate where and when PD-1 pathway signals are important for effector and memory CD8 T cell development, which interactions within the PD-1 pathway are involved and which APCs deliver these signals, and how these events are integrated with the downstream transcription factors T-bet and Eomes. We will test the hypothesis that, rather than simply inhibiting responses, the PD-1:PD-L pathway acts as a rheostat regulating cell fate decisions of CD8 T cells and long-term immunity to acute viral infection. We will test this hypothesis through the following Aims: Aim 1: To test the hypothesis that the PD-1 pathway regulates early T cell activation dynamics during priming. Aim 2: To test the hypothesis that the PD-1 pathway regulates effector CD8 T cell differentiation and function after priming, and define when and where this regulation occurs. Aim 3: To test the hypothesis that the PD-1 pathway affects CD8 T cell memory via altering homeostasis and regulating novel memory T cell subsets and Eomes. Studies in P2 will synergize with those in P1 and P3 given a major interest in P1 and P2 on subsets of memory CD8 T cells and the role of T-bet. P1, P2 and P3 also have a strong interest sin homing and migration as well as the PD-1 pathway creating a common set of shared themes. Finally, key studies on early priming events and homing and migration will be enabled through Core B.